Reverse Engineering for Beginners

CHAPTER 54. JAVA CHAPTER 54. JAVA

iload_0loads the first function argument (a),iload_2—second (b). Here is the stack after the execution of both instruc-
tions:

+---+
TOS ->| b |
+---+
| a |
+---+

iaddadds the two values and leaves the result atTOS:

+--------+
TOS ->| result |
+--------+

Let’s extend this example to thelongdata type:

public static long lsum(long a, long b)

{

return a+b;

}

...we got:

public static long lsum(long, long);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=4, locals=4, args_size=2

0: lload_0

1: lload_2

2: ladd

3: lreturn

The secondlloadinstruction takes the second argument from the 2nd slot. That’s because a 64-bitlongvalue occupies
exactly two 32-bit slots.

Slightly more complex example:

public class calc
{
public static int mult_add(int a, int b, int c)
{
return a*b+c;
}
}

public static int mult_add(int, int, int);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=3, args_size=3

0: iload_0

1: iload_1

2: imul

3: iload_2

4: iadd

5: ireturn

The first step is multiplication. The product is left at theTOS:

+---------+
TOS ->| product |
+---------+

iload_2loads the third argument (c) in the stack:

+---------+
TOS ->| c |
+---------+
| product |
+---------+

Now theiaddinstruction can add the two values.